Conclusions

In this chapter we have described the development of voxel similarity measures for intermodality medical image registration. We introduced the standard image processing approaches of image subtraction and correlation (both cross-correlation and correlation coefficient), and explained why these algorithms are inapplicable to most intermodality registration applications, despite their success in intramodality registration. We then described the variance of intensity ratios algorithm, and how the success of this algorithm for MR-PET registration inspired further research into voxel similarity measures that could be more generally applicable. The majority of the chapter was devoted to explaining how information theoretic approaches, in particular mutual information and its normalized variants, came to be widely used for intermodality registration. We described ways in which the required entropies can be estimated from the data, and suitable optimization strategies. We gave examples that illustrate both the successes of mutual information registration and its limitations.

FIGURE 6 Registration of CT images and video images by maximizing of mutual information, taken from Clarkson et al. [28]. (a) Example video image (the black circles are fiducial markers used as a gold standard to test the accuracy of the method). (b) Example rendered image. Note the absence of fiducials, which were edited out of the CT scan before rendering. (c) Misregistered overlay. (d) Registered overlay. (e) Misregistered overlay. (f) Registered overlay. The accuracy of this registration was estimated to be about 1 mm.

FIGURE 6 Registration of CT images and video images by maximizing of mutual information, taken from Clarkson et al. [28]. (a) Example video image (the black circles are fiducial markers used as a gold standard to test the accuracy of the method). (b) Example rendered image. Note the absence of fiducials, which were edited out of the CT scan before rendering. (c) Misregistered overlay. (d) Registered overlay. (e) Misregistered overlay. (f) Registered overlay. The accuracy of this registration was estimated to be about 1 mm.